1. Technical Field
The present invention relates to the field of metrology, and more particularly, to overlay metrology.
2. Discussion of Related Art
Current methods for optical overlay measurement rely on two main technologies: Imaging and Scatterometry. In imaging, the position of periodic targets is measured in the field of view of the optical system and the overlay (OVL) is deduced from positions of targets printed in different layers. Scatterometry utilizes interference between electromagnetic (EM) waves scattered by periodic overlay marks (targets with periodic structures) printed at different layers to deduce the relative displacement of the layers. In both cases a control on amplitudes and phases of the diffraction orders of the scattered EM waves may provide a crucial effect on accuracy and precision of overlay measurement.